dc.description.abstract
Incineration is considered as an environmentally friendly disposal option for waste such as municipal solid waste (MSW) or sewage sludge. One of the residues from incineration is fly ash. MSW fly ash requires special handling and disposal because of its leaching behavior.<br />Sewage sludge ash is an important secondary resource for phosphorus.<br />However, heavy metals have to be considered carefully for direct utilization as fertilizers because their concentrations are usually too high according to current regulations. Heavy metal removal from fly ash can be performed by mixing the ash with environmentally compatible chlorides (e.g.CaCl2, MgCl2) and treating the mixture at temperatures between 900 and 1100°C. In order to avoid the entrainment of significant amounts of dust in the treatment process, the mixture can be pelletized (after addition of water). The thermal treatment can be conducted in rotary or fluidized bed reactors. In this work, experiments concerning the influence of the ash type (sewage sludge ash and MSW fly ash), chloride type (CaCl2, MgCl2, NaCl), chloride amount (0 to 230 g Cl added per kilogram of ash), pelletization, reactor type (rotary reactor, fluidized bed reactor), treatment temperature (800 to 1200°C), and residence time (1 min to 20 h) on the separation efficiency for different heavy metals (Cd, Cr, Cu, Ni, Pb, Zn) were carried out. In addition, specific tests were performed in a muffle oven and by thermogravimetric analysis. During the treatment process, volatile heavy metal compounds (mainly chlorides) are formed. Depending on the type of chloride added, heavy metals are either directly (e.g.ZnO + 2 NaCl ->ZnCl2 + Na2O) or indirectly chlorinated. When chlorinated indirectly, chlorides react first with water vapor or oxygen to form HCl or Cl2 (e.g.CaCl2 + H2O -> CaO + 2 HCl); these gases subsequently chlorinate. NaCl follows the route of direct chlorination but evaporates in significant amounts without reacting as well. CaCl2 and MgCl2 are more effective for heavy metal removal; they indirectly chlorinate. Treating a pelletized mixture of sewage sludge ash and CaCl2 (150 g Cl added per kilogram of ash) in a laboratory-scale rotary reactor, at 1050°C, more than 99% of Cd, approximately 97% of Cu, 95% of Pb, and 95% of Zn can be removed after 25 min. More than 50% of these heavy metals are already volatilized during the heat-up period (i.e. after 2 min when heating from ambient temperature). For MSW fly ash, the same procedure leads to significantly lower amounts of heavy metals removed; even when using 230 g Cl per kilogram of ash, at 1050°C after 45 min approximately 95% of Cd, 60% Cu, 97% Pb, and 80% Zn can be released. These differences in heavy metal removal are caused by different heavy metal concentrations, different grain sizes, and different matrix compositions of both ashes. Without pelletization, but also depending on other conditions, higher amounts of Cu and Zn can be removed from MSW fly ash. Using CaCl2 (150 g Cl added per kilogram of ash) and a treatment temperature of 1000°C, up to 80% of Cu and Zn are volatilized after 60 min. If pelletized before treating thermally, 25% of Cu and 60% of Zn can be removed after 45 min.<br /> Further enhancement of heavy metal removal from MSW fly ash is possible by adding organic material to the ash-chloride mixture, although thermodynamic equilibrium calculations reveal that reducing conditions decrease the amount of Cu removed. Prior reduction and subsequent oxidization under chlorination condition increases the amount of Cu removed as well. For heavy metal removal from sewage sludge ash, both a rotary reactor and a fluidized bed reactor are suitable. By comparing the separation efficiency of Cu, Pb, and Zn in both reactors, the limitations of heavy metal removal concerning the external mass transfer were identified: Cu removal is limited by the evaporation and/or diffusion of its chloride; Pb and Zn removal are limited by the availability of HCl and Cl2.<br />
en